1. Field of the Invention
The present invention relates to a method of color correction which is especially applicable to a digital operation color copier. The method is based upon establishing a table which relates all of the possible input color data values that may be obtained by scanning an original image, to corresponding output signal values to be utilized in driving a printer section for printing colors respectively corresponding closely to colors of the original image.
2. Description of Prior Art
In recent years, color copiers have been developed which utilize light beam scanning of a photo-sensitive drum to form successive color images which are successively transferred to an output sheet in correct registration for producing a color image. Drive signals for this light beam scanning, referred to in the following as printer drive signals, are produced based on sequentially scanning successive portions of an original color image, separating the level of reflected light from the image into three color components (i.e. red, green, blue), converting the respective intensities of these components into respective color density values, and determining the printer drive signals in accordance with these color density values. In such a color copier apparatus, the color separation is executed by dichroic filters, and the respective levels of red, green and blue light which are detected as electrical signals during scanning of the original image are converted into successive input digital values. All subsequent operations, up to the stage of modulating the aforementioned light beams used for print-out scanning, are executed by digital processing.
The basic requirement for such a color copier apparatus is that the color of each portion of a print-out image produced from the apparatus should approach as closely as possible the color of a corresponding portion of the original image. It is possible to achieve a high degree of accuracy of color separation of the reflected light obtained by scanning the original image. However as is well known, any ink or other colorant utilized in color printing cannot provide a spectrally pure color, but is actually a mixture of colors. It is therefore necessary to execute a type of compensation for this effect, referred to as color "masking" correction, as described in detail hereinafter. In general in the prior art, this correction processing has been executed using values obtained from computations using predetermined equations in which the aforementioned input digital values are inserted as variables. However satisfactory compensation over a wide range of possible input color values cannot be achieved by such a prior art method, due to variations introduced by the characteristics of different scanner systems and printing systems, e.g. resulting from manufacturing variations. In addition, such color correction based on predetermined computations using masking equations has the disadvantage of a lack of flexibility, so that it is not possible for example to provide optimum correction within a desired specific range of color variation or range of image lightness (gray scale) variation. In addition, with such a prior art method of color masking correction, it is not possible to readily introduce desired degrees of color enhancement, i.e. to implement increased degrees of saturation of certain colors which are greater than those of the original image, or to increase the overall lightness of the output color image while maintaining chromatic values which will appear identical to those of the original image. This may be desirable, for example, to provide a more visually pleasing effect in the printed color image.